This invention relates to cellular telephony and, more particularly, to the use of packet techniques in cellular telephony.
The number of people that use cellular telephones is continually increasing. Because the available bandwidth is controlled by governmental regulations, providers of cellular telephony are meeting the increase in users by establishing smaller cell sizes. Smaller cell sizes accommodate larger numbers of mobile units within the same overall bandwidth because smaller cell sizes effectively increase the rate of bandwidth re-use per unit area. However, as cell sizes shrink, mobile units move between cells more frequently. In a circuit switched system, each move requires that one circuit be torn down and another one set up. Consequently, as cell sizes decrease, the work associated with handing off users between cells increases. In addition, when a mobile unit traverses more cells during its connection, it is more likely that the mobile unit will encounter a cell with more units than the bandwidth can support.
Packet switching, as compared to circuit switching, reduces the work required for hand off because addresses embedded within the packets are used to route individual packets rather than setting up and tearing down circuits. Packet switching was used in early military cellular systems. Those networks were designed to be rapidly deployed, were aimed primarily for wireless interconnection between mobile units, and were not connected to a wired backbone network.
Currently, the prevalent commercial cellular system in the United States is a circuit switched arrangement that employs Time Division Multiplexing (TDM). Another system, which is also a circuit switched system, employs Code Division Multiple Access (CDMA). These cellular systems can transmit data in the form of packets, but that does not constitute xe2x80x9cpacket switching,xe2x80x9d either in the sense employed in the aforementioned military system or in the sense employed in this disclosure. Specifically, while the data may have a packet format, the switching within the cellular environment is not based on the explicit address information in the packets. For example, in TDM the address is implicit in the frequency and time slot at which the mobile unit operates.
The explicit addressing characteristic of packet switching is more flexible than implicit addressing. With explicit addressing, the capacity on the shared medium can be reassigned as required and the destination can be changed without advance notice. Because of that, it is beneficial to fashion a packet switching approach for cellular communication that interfaces effectively with a wired backbone network.
An improved cellular arrangement is created with mobile units that are responsive to control signals that direct the mobile units to modify the rate of packet transmissions in case of channel overload. A mobile unit in such an arrangement includes apparatus that, when a speech signal needs to be encoded into a packet stream, encodes either the entire speech signal, or only a portion thereof that is sufficient to reproduce a reduced fidelity speech that is still intelligible. Packets that carry the portion of speech that reproduces the rediced fidelity speech are outputted by the mobile unit at a lower rate than that of packets that carry the entire speech signal.
In another embodiment, the mobile unit includes further apparatus that creates a second stream of packets, and that second stream is transmitted to a base station over a channel that is different from the channel over which the first stream of packets is transmitted. The second stream of packets, also having a lower rate than that of packets that carry the entire speech signal, complements the first stream of packets in its lower rate form, to allow reproducing the speech signal with high fidelity.
Operationally, such a mobile unit is responsive to control signals from the base station. When the base station decides that a primary channel over which the mobile unit is communicating with the base station is too heavily loaded, it sends a command to the mobile unit to transmit packets at the lower rate (and the reduced fidelity). Optionally, the base station can also direct the mobile unit to send the second stream of packets over a different channel, e.g., to a different base station.